This invention relates to methods for curing resins that are used as binders or adhesives for materials such as wood particles, wood chips, wood wafers, wood strips, wood fibres and wood veneers, for example. The invention is particularly applicable to the production of chip board, hard board, particle board, wafer board, plywood and products of the type described in Canadian Patent No. 966,409 issued on Apr. 22nd, 1975, and assigned to MacMillan Bloedel Limited, but it is to be understood at the outset, even though the subsequent disclosure will be with respect to wood-glue mixtures, that other materials commonly used as glue fillers, and reinforcing materials such as fiberglass, metal, cement, etc., in whatever form they may take, can be included.
Wood products of the type hereinbefore noted classically are made by being subjected to heat and pressure in a hot press. This process is time consuming and hence costly. For example, 13/16 inch thick sheet of exterior grade plywood composed of seven plies of Douglas fir veneer dried to less than 7% moisture content and glued together with 56 lbs. of phenol formaldehyde glue per 1000 sq. ft. of double glue line typically requires a residence time of 71/2 min. at 200 p.s.i. and 300.degree. F. The time consuming nature of the process arises from the fact that wood is a relatively poor conductor of heat, and the heat from the platens of a hot press can only be directed against the outer surfaces of the wood product being formed. Consequently, considerable time is required for the amount of heat necessary to cure the resin to penetrate to the centre of the wood product being formed. While it may be possible to reduce the time consumed by increasing the platen temperature, there is an obvious temperature limit imposed by the necessity to avoid scorchng or charring the outer surfaces of the wood product being formed. In addition, higher temperatures may be more difficult and expensive to obtain requiring greater steam pressures and additional equipment. It also should be noted that at higher temperatures water may be entrapped causing stream explosions.
In an effort to reduce the time required to cure the resin, numerous attempts have been made using R.F. energy, i.e., dielectric heating. In many cases, where the resin is in layers, as in plywood, for example, the parallel heating techniques has been employed because the resin tends to heat preferentially to the wood. In this technique the electric field is parallel to the glue line. It has been observed repeatedly, where the parallel heating technique has been used, and where the resin has been an alkaline solution of phenol formaldehyde resin, that arcing and tracking in the resin takes place, and that the problem is more acute the thicker the resin layer. The reason for this undesirable phenomenon appears to be the relatively high conductivity of such resins which leads to breakdowns when subjected to R.F. fields having the strength required to obtain relatively short cure times. The arcing and tracking phenomenon can be reduced and perhaps even eliminated if the R.F. field is applied transverse to the glue line, but this reduces the efficiency of the operation because of the increased energy required per unit volume, this being due to the necessity to heat fully both the wood and the resin, and increases the cost of the process.
It also has been observed that reasonable curing times using R.F. energy can be obtained where acidic phenol formaldehyde resins are employed, but these resins have inferior ageing properties, at least in many cases.
Likewise the R.F. arcing problem has been shown to be reduced if a resorcinol resin is added to the alkaline phenolic resin, but this undesirably increases the overall resin cost.
As a consequence of their characteristics and cost, phenol formaldehyde resins dissolved in aqueous alkaline solutions hereinafter called alkaline phenolic resins, are in widespread use throughout the world in wood-resin products of the type noted beforehand. The problem of satisfactorily curing these resins in a shorter time than is possible in a hot press remains extant.